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ART CBOR Concise Data Definition Language The Concise Data Definition Language (CDDL), as defined in RFC 8610 and RFC 9165, provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON. The present document updates RFC 8610 by addressing errata and making other small fixes for the ABNF grammar defined for CDDL there. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the CBOR Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The Concise Data Definition Language (CDDL), as defined in and , provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON. The present document updates by addressing errata and making other small fixes for the ABNF grammar defined for CDDL there.

Conventions and Definitions The Terminology from applies. The grammar in is based on ABNF, which is defined in and .

Clarifications and Changes based on Errata Reports A number of errata reports have been made around some details of text string and byte string literal syntax: and . These are being addressed in this section, updating details of the ABNF for these literal syntaxes. Also, needs to be applied (backslashes have been lost during RFC processing in some text explaining backslash escaping). These changes are intended to mirror the way existing implementations have dealt with the errata. They also use the opportunity presented by the necessary cleanup of the grammar of string literals for a backward compatible addition to the syntax for hexadecimal escapes. The latter change is not automatically forward compatible (i.e., CDDL specifications that make use of this syntax do not necessarily work with existing implementations until these are updated, which this specification recommends).

Err6527 (text string literals) The ABNF used in for the content of text string literals is rather permissive:

Old ABNF for strings with permissive ABNF for SESC, but not allowing hex escapes

This allows almost any non-C0 character to be escaped by a backslash, but critically misses out on the \uXXXX and \uHHHH\uLLLL forms that JSON allows to specify characters in hex (which should be applying here according to Bullet 6 of ). (Note that we import from JSON the unwieldy \uHHHH\uLLLL syntax, which represents Unicode code points beyond U+FFFF by making them look like UTF-16 surrogate pairs; CDDL text strings are not using UTF-16 or surrogates.) Both can be solved by updating the SESC production. We use the opportunity to add a popular form of directly specifying characters in strings using hexadecimal escape sequences of the form \u{hex}, where hex is the hexadecimal representation of the Unicode scalar value. The result is the new set of rules defining SESC in :

Updated string ABNF to allow hex escapes

(Notes: In ABNF, strings such as "A", "B" etc. are case-insensitive, as is intended here. We could have written %x62 as %s"b", but didn't, in order to maximize ABNF tool compatibility.) Now that SESC is more restrictively formulated, this also requires an update to the BCHAR production used in the ABNF syntax for byte string literals:

Old ABNF for BCHAR

With the SESC updated as above, \' is no longer allowed in BCHAR; this now needs to be explicitly included. Updating BCHAR also provides an opportunity to address , which points to an inconsistency in treating U+007F (DEL) between SCHAR and BCHAR. As U+007F is not printable, including it in a byte string literal is as confusing as for a text string literal, and it should therefore be excluded from BCHAR as it is from SCHAR. The same reasoning also applies to the C1 control characters, so we actually exclude the entire range from U+007F to U+009F. The same reasoning then also applies to text in comments (PCHAR). For completeness, all these should also explicitly exclude the code points that have been set aside for UTF-16's surrogates.

Updated ABNF for BCHAR, SCHAR, and PCHAR

(Note that, apart from addressing the inconsistencies, there is no attempt to further exclude non-printable characters from the ABNF; doing this properly would draw in complexity from the ongoing evolution of the Unicode standard that is not needed here.)

Err6543 (byte string literals) The ABNF used in for the content of byte string literals lumps together byte strings notated as text with byte strings notated in base16 (hex) or base64 (but see also updated BCHAR production above):

Old ABNF for BCHAR

Change proposed by Errata Report 6543 Errata report 6543 proposes to handle the two cases in separate productions (where, with an updated SESC, BCHAR obviously needs to be updated as above):

Errata Report 8653 Proposal to Split the Byte String Rules

This potentially causes a subtle change, which is hidden in the WS production:

ABNF definition of WS from RFC 8610

This allows any non-C0 character in a comment, so this fragment becomes possible: The current text is not unambiguously saying whether the three apostrophes need to be escaped with a \ or not, as in: ... which would be supported by the existing ABNF in .

No change needed after addressing (Section ) This document takes the simpler approach of leaving the processing of the content of the byte string literal to a semantic step after processing the syntax of the bytes/BCHAR rules as updated by and . The rules in are therefore applied to the result of this processing where bsqual is given as h or b64. Note that this approach also works well with the use of byte strings in . It does require some care when copy-pasting into CDDL models from ABNF that contains single quotes (which may also hide as apostrophes in comments); these need to be escaped or possibly replaced by %x27. Finally, our approach lends support to extending bsqual in CDDL similar to the way this is done for CBOR diagnostic notation in . (Note that the processing of string literals now is quite similar between CDDL and EDN, except that CDDL has ";"-based end-of-line comments, while EDN has two comment syntaxes, in-line "/"-based and end-of-line "#"-based.) The CDDL example in demonstrates various escaping techniques. Obviously in the literals for a and x, there is no need to escape the second character, an o, as \u{6f}; this is just for demonstration. Similarly, as shown in c and z there also is no need to escape the 🁳 or ⌘, but escaping them may be convenient in order to limit the character repertoire of a CDDL file itself to ASCII .

Example text and byte string literals with various escaping techniques

In this example, the rules a to c and x to z all produce strings with byte-wise identical content, where a to c are text strings, and x to z are byte strings. illustrates this by showing the output generated from the start rule in , using pretty-printed hexadecimal.

Generated CBOR from CDDL example

Small Enabling Grammar Changes The two subsections in this section specify two small changes to the grammar that are intended to enable certain kinds of specifications. These changes are backward compatible, i.e., CDDL files that comply to continue to match the updated grammar, but not necessarily forward compatible, i.e., CDDL specifications that make use of these changes cannot necessarily be processed by existing implementations.

Empty data models requires a CDDL file to have at least one rule.

Old ABNF for top-level rule cddl

This makes sense when the file has to stand alone, as a CDDL data model needs to have at least one rule to provide an entry point (start rule). With CDDL modules , CDDL files can also include directives, and these might be the source of all the rules that ultimately make up the module created by the file. Any other rule content in the file has to be available for directive processing, making the requirement for at least one rule cumbersome. Therefore, we extend the grammar as in and make the existence of at least one rule a semantic constraint, to be fulfilled after processing of all directives.

Updated ABNF for top-level rule cddl

Non-literal Tag Numbers, Simple Values The existing ABNF syntax for expressing tags in CDDL is:

Old ABNF for tag syntax

This means tag numbers can only be given as literal numbers (uints). Some specifications operate on ranges of tag numbers, e.g., has a range of tag numbers 1668546817 (0x63740101) to 1668612095 (0x6374FFFF) to tag specific content formats. This can currently not be expressed in CDDL. Similar considerations apply to simple values (#7.xx). This update extends the syntax to:

Updated ABNF for tag and simple value syntaxes ") ]]>

For #6, the head-number stands for the tag number. For #7, the head-number stands for the simple value if it is in the ranges 0..23 or 32..255 (as per Section of RFC 8949 the simple values 24..31 are not used). For 24..31, the head-number stands for the "additional information", e.g., #7.25 or #7.<25> is a float16, etc. (All ranges mentioned here are inclusive.) So the above range can be expressed in a CDDL fragment such as: = #6.(content) ct-tag-number = 1668546817..1668612095 ; or use 0x63740101..0x6374FFFF ]]> Notes:

1. This syntax reuses the angle bracket syntax for generics; this reuse is innocuous as a generic parameter/argument only ever occurs after a rule name (id), while it occurs after . here. (Whether there is potential for human confusion can be debated; the above example deliberately uses generics as well.)
2. The updated ABNF grammar makes it a bit more explicit that the number given after the optional dot is special, not giving the CBOR "additional information" for tags and simple values as it is with other uses of # in CDDL. (Adding this observation to is the subject of ; it is correctly noted in .) In hindsight, maybe a different character than the dot should have been chosen for this special case, however changing the grammar now would have been too disruptive.

Security Considerations The grammar fixes and updates in this document are not believed to create additional security considerations. The security considerations in do apply, and specifically the potential for confusion is increased in an environment that uses a combination of CDDL tools some of which have been updated and some of which have not been, in particular based on .

IANA Considerations This document has no IANA actions.

References Normative References This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Informative References This document extends the base definition of ABNF (Augmented Backus-Naur Form) to include a way to specify US-ASCII string literals that are matched in a case-sensitive manner. The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. The present document defines a number of control operators that were not yet ready at the time RFC 8610 was completed:,, and for the construction of constants; / for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and for indicating the use of a non-basic feature in an instance. Universität Bremen TZI Arm Limited At the time of writing, the Concise Data Definition Language (CDDL) is defined by RFC 8610 and RFC 9165. The latter has used the extension point provided in RFC 8610, the _control operator_. As CDDL is being used in larger projects, the need for features has become known that cannot be easily mapped into this single extension point. The present document defines a backward- and forward-compatible way to add a module structure to CDDL. Universität Bremen TZI The Concise Binary Object Representation, CBOR (STD 94, RFC 8949), defines a "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. This document specifies how to add application-oriented extensions to the diagnostic notation. It then defines two such extensions for text representations of epoch-based date/times and of IP addresses and prefixes (RFC 9164). A few further additions close some gaps in usability. To facilitate tool interoperation, this document specifies a formal ABNF definition for extended diagnostic notation (EDN) that accommodates application- oriented literals. This document defines a stored ("file") format for Concise Binary Object Representation (CBOR) data items that is friendly to common systems that recognize file types, such as the Unix file(1) command.

Updated Collected ABNF for CDDL This appendix is normative. It provides the full ABNF from with the updates applied in the present document.

ABNF for CDDL as updated " genericarg = "<" S type1 S *("," S type1 S ) ">" type = type1 *(S "/" S type1) type1 = type2 [S (rangeop / ctlop) S type2] ; space may be needed before the operator if type2 ends in a name type2 = value / typename [genericarg] / "(" S type S ")" / "{" S group S "}" / "[" S group S "]" / "~" S typename [genericarg] / "&" S "(" S group S ")" / "&" S groupname [genericarg] / "#" "6" ["." head-number] "(" S type S ")" / "#" "7" ["." head-number] / "#" DIGIT ["." uint] ; major/ai / "#" ; any head-number = uint / ("<" type ">") rangeop = "..." / ".." ctlop = "." id group = grpchoice *(S "//" S grpchoice) grpchoice = *(grpent optcom) grpent = [occur S] [memberkey S] type / [occur S] groupname [genericarg] ; preempted by above / [occur S] "(" S group S ")" memberkey = type1 S ["^" S] "=>" / bareword S ":" / value S ":" bareword = id optcom = S ["," S] occur = [uint] "*" [uint] / "+" / "?" uint = DIGIT1 *DIGIT / "0x" 1*HEXDIG / "0b" 1*BINDIG / "0" value = number / text / bytes int = ["-"] uint ; This is a float if it has fraction or exponent; int otherwise number = hexfloat / (int ["." fraction] ["e" exponent ]) hexfloat = ["-"] "0x" 1*HEXDIG ["." 1*HEXDIG] "p" exponent fraction = 1*DIGIT exponent = ["+"/"-"] 1*DIGIT text = %x22 *SCHAR %x22 SCHAR = %x20-21 / %x23-5B / %x5D-7E / NONASCII / SESC SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\ %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t (%x75 hexchar) ) ; \uXXXX hexchar = "{" (1*"0" [ hexscalar ] / hexscalar) "}" / non-surrogate / (high-surrogate "\" %x75 low-surrogate) non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) / ("D" %x30-37 2HEXDIG ) high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG hexscalar = "10" 4HEXDIG / HEXDIG1 4HEXDIG / non-surrogate / 1*3HEXDIG bytes = [bsqual] %x27 *BCHAR %x27 BCHAR = %x20-26 / %x28-5B / %x5D-7E / NONASCII / SESC / "\'" / CRLF bsqual = "h" / "b64" id = EALPHA *(*("-" / ".") (EALPHA / DIGIT)) ALPHA = %x41-5A / %x61-7A EALPHA = ALPHA / "@" / "_" / "$" DIGIT = %x30-39 DIGIT1 = %x31-39 HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" HEXDIG1 = DIGIT1 / "A" / "B" / "C" / "D" / "E" / "F" BINDIG = %x30-31 S = *WS WS = SP / NL SP = %x20 NL = COMMENT / CRLF COMMENT = ";" *PCHAR CRLF PCHAR = %x20-7E / NONASCII NONASCII = %xA0-D7FF / %xE000-10FFFD CRLF = %x0A / %x0D.0A ]]>

Acknowledgments Many thanks go to the submitters of the errata reports addressed in this document. In one of the ensuing discussions, proposed to define an ABNF rule NONASCII, of which we have included the essence. Special thanks to the reviewers , (shepherd review), and (AD review).